I just went through this myself, and purchased 3585A myself because of the lower cost. There isn't much at all different in their features or operation other than a few upgraded specs on the B model. The biggest difference is processing speed. Like the 856X analyzers this model got a processor enhancement with the B suffix (HP custom processor to Motorola if the same as the 856X analyzers) that speeds it up. The B sells for a lot more, so its a tough decision if its worth it or not. The A is sort of slow processing low frequencies at low RBW's, but it will do the job.
Forgot to mention to eat your Wheaties before picking it up. These analyzers are HEAVY.
-Mark-